Historically, automatic lateral-directional systems for controlling an aircraft's horizontal flight path and sideslip angle have been developed in a piecemeal fashion, with additional functional capabilities being added one at a time. One of the early developments was a control mode in which a commanded roll attitude of the aircraft is automatically attained and held. Following the development of this type of control mode, successive control modes were added to automatically select and hold the aircraft's heading, control the aircraft by means of a localizer (a guidance signal from an airport), and finally to control the aircraft in a lateral navigation mode. The last mentioned control mode is referred to as "waypoint steering" and determines the aircraft's flight path on the basis of a series of flight legs, each of which is defined by two points on the ground. In the development of automatic lateral-directional control systems for aircraft, yaw damping and turn coordination capabilities were largely developed separately from the control modes discussed above.
The piecemeal development of lateral-directional automatic control systems has led to a number of disadvantages and problems in the known state of the art. Known control systems are overly complex in terms of both hardware and software. Because the different control modes and functional capabilities have been developed separately, rather than as part of an integrated system, known lateral-directional control systems tend to provide inconsistent performance and to have inconsistent stability characteristics between various control modes and various flight conditions. In addition, known systems fail to provide certain desirable capabilities. These include active sideslip control (i.e. active control of motion of the aircraft in a lateral direction relative to the longitudinal axis of the aircraft); automatic roll/yaw trim (i.e. automatic positioning of control surfaces to maintain the aircraft's horizontal flight path, for conditions such as engine failure); automatic flat turn capability; and automatic forward sideslip decrab for crosswind landing.
What is meant by "decrab" is the last minute lining up of the longitudinal axis of the aircraft with the runway just before touchdown. This is necessary because of the conventional technique of crabbing the aircraft in a crosswind, i.e. orienting the nose of the aircraft into the crosswind to give it an apparent sideways motion with respect to the ground and compensate for the crosswind. When an aircraft must land in a relatively high crosswind, landing in a crabbed orientation places severe stresses on the landing gear. In addition, landing in a crabbed orientation can result in the aircraft rolling off the runway. Therefore, it is desirable to realign the aircraft with the runway prior to touchdown.
An undesirable consequence of the inconsistent performance and stability characteristics of known lateral-directional automatic pilot control systems is that known systems are not well-suited for use on autonomous pilotless vehicles. Such vehicles must function properly under all flight conditions without pilot supervision or assistance. It is desirable to maintain proper functioning even under severely adverse conditions, such as engine failure.
The history of the development of lateral-directional control systems has also resulted in each control mode tending to have its own peculiar design and performance characteristics. The inconsistency in design and performance characteristics from mode to mode makes it difficult to integrate the control modes into a system. Moreover, problems in system integration are difficult to detect and have often only been detected at a late stage in the development program of a system. When the problems arise very late in a program in the flight test phase, the risk of damage to test aircraft and danger to test pilots is added to the disadvantages of high cost and loss of time caused by delays in discovering faults in the system.
Because known lateral-directional control systems lack a consistent overall design philosophy, adding a new function to such a system or adapting it to a new aircraft is very costly and time consuming. Extensive basic engineering and system integration efforts are required. In addition, each function must be separately flight tested, and the process of certifying the system for a particular aircraft is complex. All this results in a very high cost for engineering development and maintenance of known systems.
As discussed above, to the best of the applicant's knowledge, there are no known lateral-directional automatic control systems for aircraft that are fully integrated and that provide a consistent design philosophy for all control modes and flight conditions. U.S. Pat. No. 4,536,843, granted Aug. 20, 1985, to the present applicant, discloses an integrated system for controlling speed and vertical flight path of an aircraft. In the patented system, cross-over inputs from flight path to the thrust control and from speed to the elevator control are employed to obtain simultaneous speed and flight path control based on kinetic and potential energy principles. The system generates a total energy rate error signal and an energy rate distribution error signal, each of which has a flight path component and a speed component. The aircraft thrust control is operated to control the total energy state and reduce the total energy rate error to zero. The elevator control is simultaneously operated to control the distribution of energy between potential energy (altitude) and kinetic energy (airspeed) and reduce the energy rate distribution error to zero. The system provides integrated speed and vertical flight path control and helps eliminate undesired throttle activity to enhance fuel efficiency. As disclosed in the patent, the system reduces speed and flight path errors to zero at the same rate and specifically avoids coupling flight path control and speed control. In other words, adjustments to correct speed errors do not create errors in flight path and vice versa.